88 lines
1.7 KiB
C
88 lines
1.7 KiB
C
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#include <inttypes.h>
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#define UART0_BASE 0x81000000
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#define UART0_SR (*((volatile uint32_t *) (UART0_BASE + 0x00)))
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#define UART0_DR (*((volatile uint32_t *) (UART0_BASE + 0x04)))
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#define UART0_SR_RX_DATA_EMPTY (1 << 0)
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#define UART0_SR_TX_FULL (1 << 1)
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/*
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// Set up stack pointer
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asm("_start:\
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xor sp, sp, sp;\
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lui sp, 0x80100;\
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j main;\
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");
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*/
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__attribute__((noreturn)) void _start() {
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uint8_t state = 0;
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uint8_t c;
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uint8_t opcode;
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uint8_t *ptr;
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uint32_t length;
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/*while (1) {
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while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
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c = UART0_DR;
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while (UART0_SR & UART0_SR_TX_FULL);
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UART0_DR = c;
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}*/
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while (1) {
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while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
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c = UART0_DR;
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state++;
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switch (state) {
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case 1: // Opcode
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opcode = c;
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ptr = 0;
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length = 0;
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if (c == 0) // NOP
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state = 0;
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break;
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case 2: // Address
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case 3:
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case 4:
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case 5:
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ptr = (uint8_t*)(((uint32_t)ptr << 8) | c);
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if (state != 5)
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break;
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if (opcode == 3) { // Jump
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//((void (*)()) ptr)();
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asm("jalr %0" : : "r"(ptr)); // Not sure why, but the jump above causes GCC to save variables to the stack at the beginning of this function
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__builtin_unreachable();
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}
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break;
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case 6: // Length
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case 7:
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case 8:
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case 9:
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length = (length << 8) | c;
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if (state != 9)
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break;
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if (opcode == 2) { // Read
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for (; length > 0; length--) {
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while (UART0_SR & UART0_SR_TX_FULL);
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UART0_DR = *(ptr);
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ptr++;
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}
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state = 0;
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}
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break;
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case 10: // Write byte by byte
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*(ptr++) = c;
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if (--length == 0) {
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state = 0;
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} else {
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state--; // Stay in this state
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}
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break;
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default:
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break;
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}
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}
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__builtin_unreachable();
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}
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